Thymus derived lymphocytes (T-cells) and cytotytic antibody have been shown to be effective in destruction and rejection of cellular antigens including some protozoan parasites. The importance of these mechanisms in rejection of Trypanosoma cruzi by infected mammalian hosts is unresolved. An in vitro technique similar to those used to investigate lymphocyte mediated cytotoxicity in tumor and transplant systems has recently been developed in our laboratory for studies on immunity in experimental Chagas' disease. Trypanosomes grown in the presence of 14C-amino acids incorporate the isotope into membrane and cytoplasmic elements. When specific lymphocyte or antibody mediated lysis occurs, the parasite releases 14C-labeled cytoplasmic materials into the surrounding medium. The amount of 14C-released is a quantitative measure of parasite destruction. Using this technique, experiments are proposed which will examine the capacity of mice to develop lymphocyte mediated cytotoxicity (LMC) and cytolytic antibody against T. cruzi. The development of LMC and cytolytic antibody will be determined during the course of the disease and as a result of immunizations with irradiated parasites and antigenic extracts. Extracts and more purified trypanosome fractions shown in vitro to be effective in eliciting high levels of LMC and/or cytolytic antibody will be tested in vivo for induction of protective immuunity. Additional studies are designed to determine the role of T cells in anti-trypanosome cytotoxic responses, the interaction of antibody and lymphocytes in killing parasites and the antigenic specificity of these cytotoxic responses.